This invention relates to improvements in interference fit, nondeformable metal, headed round pin-type fasteners cooperable with structural members of relatively deformable metal to be fastened thereby, the most important present use of the invention being in aircraft structural joints. More specifically the invention relates to high strength, lightweight fasteners for applications in which maximum resistance to fatigue failure is a vital requirement. The invention is herein illustratively described in reference to the presently preferred embodiments thereof; however, it will be recognized that certain modification and changes therein with respect to details may be made without departing from the essential features involved.
Prior art patents disclosing fasteners of varying degrees of background interest to the present invention are listed below.
Schmitt, U.S. Pat. No. 3,821,871 PA0 Schmitt, U.S. Pat. No. 3,828,422 PA0 Schmitt, U.S. Pat. No. 3,748,948 PA0 Briles, U.S. Pat. No. 3,680,429 PA0 Briles, U.S. Pat. No. 4,050,833 PA0 Speakman, U.S. Pat. No. 3,779,127 PA0 Rosman, U.S. Pat. No. 3,747,467 PA0 Diemer, U.S. Pat. No. 3,561,102 PA0 Auriol, U.S. Pat. No. 3,840,980 PA0 Cautley, U.S. Pat. No. 2,018,913
The above listed patents are directed to fasteners generally of background interest to this invention inasmuch as they illustrate prior art concepts for increasing the strength of durability of a joint through deformation and stressing of joint components. These fasteners are of two basic types. In one, the hole wall material is deformable relative to the hard body of the fastener which is made larger than the hole so as to produce what is termed an interference fit placing the hole wall material under permanent stress. With such fasteners, precise control of final stress and stress distribution in the hole wall material, i.e., in the structural members in which the hole is formed, is critical to the performance characteristics of the joint. The original shape of the fastener being maintained throughout the installation, the fastener profile in relation to original hole profile becomes the control reference upon the basis of which the stress pattern in the structural parts can be reliably predicted and consistently reproduced with the necessary precision.
In the second basic type of fastener illustrated in the above listed patents, the so-called rivet principle is employed wherein the fastener itself being of deformable material undergoes a change of shape during or upon completion of the installation. In some cases the structural parts being joined may also undergo deformation.
The present invention is of the first mentioned type, namely, a nondeformable, interference fit, precision stress control fastener. It is typically useful in aircraft structural joints, an example being in the connections between the aluminum alloy wing skins and wing stiffeners which are subjected to repeated cycles of loading during flight. In such applications, the fasteners must be light in weight and strong, typical fastener materials being 6Al-4 V, titanium and high strength steels. The hardness and strength of such materials used in these interference fit fasteners provide a basic assurance of factor of safety, both in static load and in fatigue ratings of the fastener itself as the basic primary joint component. Consequently, the limiting factor on the strength and fatigue durability of structural joints using such fasteners lies in the fastened condition of the structural members secured together. This fatigue limitation has been observed through fatigue rating tests and other observations based on a variety of fasteners and joints formed with them.
Accordingly, a long standing objective of fastener technology and a basic purpose of this invention has been to improve the fatigue characteristics of such fastener joints, and particularly the fatigue durability of the structural part material surrounding the fastener, especially that at the end of the fastener hole beneath the fastener head. It is observed that many fatigue failures that do occur tend to originate at this location. And it is also in the region of the fastener head where corrosion problems seem to concentrate when they do arise with untreated fastener and/or wall material surfaces. The two problems are found to be traceable to undesirable residual stress conditions and stress discontinuities in the wall material surrounding the hole entrance. Moreover, in some prior fasteners using tapered shanks installed in straight holes, warpage and bowing of structural members caused by fastener-induced stresses have occurred. While great strides have been made in interference fit fastener technology by the aircraft industry, the critical nature of the subject imposes its own high priority on the continued search for still further improved fasteners. Accordingly, while prior art contributions, such as those of the above listed Schmitt patents, have played a major role in attaining the durable safety of aircraft structural joints realized in current aircraft construction, it is an object hereof to achieve still further improvements therein and to do so by a precision stress control fastener design radily suited to present day manufacturing capabilities of the aircraft industry.
Yet another object hereof is to devise an improved interference fit stress control fastener and fastener joint incorporating the same wherein any remaining problem tendency of prior art joints to allow cracks to start around the hole entrance at the head end of the fastener are substantially reduced.
A related object hereof is to achieve a fastener joint with reduced tendency for corrosion to occur in the joint surfaces of the fastener head and surrounding wall material.
Still another object hereof is to devise such fastener and joint improvements applicable both to protruding head type fasteners and to flush head type fasteners in which the fastener hole is conically countersunk and the underside of the fastener head is substantially matched in nominal shape.
It is also an object of this invention to provide an improved fastener and joint using the same that reduces any tendency for galling and scoring of the interference fit hole wall, or of excessively working the wall material being "coined" by fastener insertion through an elongated straight cylindrical hole with an interference fit fastener.
A specific object hereof is to so modify and improve the stress distribution pattern in the hole wall material surrounding a structural joint fastener as to avoid dynamic load stress concentrations in such material that have the potential to start localized fatigue failures after prolonged load cycling of the joint; also to avoid the tendency with certain prior art fasteners using tapered shanks installed in straight holes to induce bowing stresses or warpage in one of the joined structural members.
A further object hereof is to devise an improved interference fit stress control fastener for straight cylindrical holes that substantially avoids or eliminates the previously experienced adverse effects of hole wall galling, scoring and damaage due to the fastener expanding the hole during entry and/or being cocked during initial installation.